Prosthesis for the replacement of hip joints are well known. Originally, only the ball-end on the head of the femur could be replaced but it has since proved possible to replace either part of the hip joint, i.e., the acetabular socket of the joint or the ball-end on the femur.
Known acetabular cup implants, which form the socket portion of an artificial hip joint, comprise a metal cup shell, which is secured within the acetabulum in the pelvic bone of a patient, and an inner liner of plastic material that provides a spherical bearing surface for receiving the ball portion of the joint. The metal cup shell may be provided with an external thread to facilitate anchorage to the pelvic bone or may be secured by other means such as cement or screws.
Metal cup shells having an external screw thread are widely used and are disclosed, for example, in U.S. Pat. Nos. 3,894,297 and 4,662,891 and International Publication No. WO 87/05490. However it is often difficult to fully insert metal cup shells having self tapping screw threads, as is necessary to ensure contact of the entire surface area of the outside of the metal cup with the surface of the acetabulum to provide maximum transfer of forces from the prosthesis to the pelvic bone. The implants often have a tendency to jam, and great force is necessary to rotate the implant in either direction, which may cause bone fracture.